/Engine/Source/Runtime/Core/Public/PixelFormat.h
Get 2D/3D image/texture size in bytes. This is for storage of the encoded image data, and does not adjust for any GPU alignment/padding constraints. It is also not valid for tiled or packed mip tails (i.e. cooked mips for consoles). Only use these when you know you're working with bog standard textures/images in block based pixel formats.
uint64 Get2DImageSizeInBytes ( uint32 InWidth, uint32 InHeight ) const